The production of polyurethane urea elastomers from polyisocyanates, relatively high molecular weight polyhydroxyl compounds and aromatic diamines is known. In order to obtain acceptable processing times for reactive systems from such starting components, the reactive aromatic isocyanates (which are the ones most commonly used industrially) are advantageously reacted with relatively inert diamines. The diamines which have proved to be particularly suitable for this purpose are those aromatic diamines in which the basicity and hence the reactivity with isocyanates has been reduced. This lowering of the reactivity may be achieved by the introduction of suitable "obstructive" substituents which lower the nucleophilic character or have a steric action. Examples of such substituents include halogen: e.g., as in 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA) or 3,5-diamino-4-chloro-1-alkylbenzene (German Offenlegungsschrift No. 2,065,869): carboxylic acid esters: 3,5-diamino-4-chloro-benzoic acid alkyl esters (German Offenlegungsschrift No. 2,003,706) or 4,4'-diamino-3,3'-carbethoxy-diphenylmethane (French Pat. No. 2,104,442); sulphonic acid amides: 3,5-diamino-4-alkyl-benzenesulphonic acid dibutylamide (European Pat. No. 0,037,029); S, S-S or S-R-S: 2,2'-diaminodiphenyldisulphide (German Offenlegungsschrift No. 2,435,872) or 1,2-bis-(2-aminophenylmercapto)-ethane (U.S. Pat. No. 3,920,617); and alkyl groups: 4,4'-diamino-3,3',5,5'-tetramethyldiphenylmethane (German Offenlegungsschrift No. 2,731,815) or 3,5-diethyl-2,4-diaminotoluene.
The aromatic chain lengthening agents mentioned above, however, have only a limited, relatively short casting and pot life when used in combination with isocyanate-prepolymers and must therefore be used very quickly, especially at elevated temperatures. In many cases, the substituents required for reducing the amine reactivity "interfere" with the structures of the hard segments produced with the isocyanates, with the result that the reduced amine reactivity may be inevitably accompanied by a deterioration in the properties of the polyurethanes.
U.S. Pat. No. 3,891,606 discloses the crosslinking of isocyanate prepolymers obtained from polyhydroxyl compounds and an excess of polyisocyanates with aromatic diamines whose reactivity with isocyanate groups has been reduced by complex formation with certain alkali metal salts. The disadvantage of this method is that it is limited to two particular aromatic diamines. Moreover, complex formation between the aromatic diamine and the alkali metal salt must be carried out in a separate process step.
Little work has been done relative to the production of polyurethane ureas by the reaction of relatively high molecular weight polyisocyanates with aromatic diamines in heterogeneous phase.
German Auslegeschrift No. 1,122,699 relates to a process for the production of polyurethane elastomers by cross-linking liquid isocyanate prepolymers by a reaction with mixtures of primary diamines and compounds containing several hydroxyl groups, the cross-linking being accompanied by shaping. In this process, a dispersion of a pulverulent, crystalline diamine in a liquid polyester or polyether containing several hydroxyl groups or in castor oil is introduced into the prepolymer at a temperature below the melting point of the diamine. The mass is hardened in the mixture at temperatures above the melting point of the diamine.
A process for the production of polyurethane ureas is described in U.S. Pat. No. 3,105,062. In this process, an aromatic diamine is added in the form of a finely milled powder to an isocyanate prepolymer. The reaction mixture obtained, which is initially a two-phase system, changes into a reaction melt only in the course of solidification at elevated temperature. Products which are not homogeneous in composition are frequently obtained due to lack of sufficiently thorough mixing.
In the processes according to German Auslegeschrift No. 1,122,699 and U.S. Pat. No. 3,105,062, "amine cross-linking" takes place in the liquid phase at the appropriate temperatures. The disadvantage of these processes is the high temperature required for converting the components into the liquid state, especially when high melting diamines are used, such as 1,5-naphthylene diamine (m.p.=189.degree. C.) or 4,4'-diaminodiphenylether (m.p.=186.degree. C.). It has been found that considerable decomposition reactions occur in the polyurethane under these conditions, with the result that the mechanical properties of the end products undergo changes which cannot be controlled. Moreover, the storage stability of these heterogeneous reaction mixtures is very limited at room temperature, amounting only to several hours. Depending on the choice of the aromatic diamine and especially of the relatively high molecular weight polyisocyanate (isocyanate prepolymer based on polyether or polyester), a continuous increase in viscosity takes place and the substance changes into a pasty form and finally to a solid phase which can no longer be used.
The encapsulation of amines is known, e.g., from U.S. Pat. Nos. 3,396,116 and 3,396,117. The wall of the capsule or shell consists of water-soluble alginates, optionally in combination with other polymers such as polyvinyl alcohol, polyethylene oxides, polyacrylates or gelatin, which may subsequently be afterhardened.
According to U.S. Pat. No. 3,384,680, liquid polyamines are bound to finely divided, absorbent materials and subsequently sealed and used for epoxide formulations.
According to U.S. Pat. No. 3,791,980, electrically conductive amines, e.g., diethylamine, are extruded under high tension through nozzles to form charged, round droplets which are then enclosed in liquid, fusible material.
German Offenlegungsschrift No. 3,224,456 describes a microencapsulation of water-insoluble, liquid polyamines (optionally in the presence of water-soluble protective colloids) with water-soluble bisulphite adducts of polyisocyanates in aqueous solution, and the use of such microencapsulated, liquid polyamines in the preparation of polyurethane resin formulations.
German Offenlegungsschrift No. 3,224,454 describes the microencapsulation of water-insoluble substances (e.g., color producing components for reactive carbon paper) by encapsulation with polyurea from liquid polyamines and water-soluble polyisocyanate derivatives in aqueous solution, optionally in the presence of protective colloids (see also U.S. Pat. Nos. 3,575,882, 3,607,776 and 4,076,774 and German Offenlegungsschrift No. 2,655,048).
Finally, a process for the preparation of encapsulated aromatic diamines is described in Soviet Pat. No. 535,289. In this process, the aromatic diamine used is subjected to an interaction with subequivalent quantities of diisocyanate in the presence of a 6:1 hexane/ethyl acetate mixture at room temperature. These "encapsulated diamines" were claimed as hardeners for epoxide resins, but this literature does not show to what extent the polyamines, which are modified in a special solvent mixture and must first be isolated from them, are reduced in their isocyanate reactivity.